1. Field of the Invention
The present invention relates to an ultrasonic testing method and equipment therefor capable of measuring an area of a defect extending in the rolling direction of a test object with high precision and simply.
2. Description of the Related Art
To guarantee the quality of a rolled metal product (including half-finished products) such as iron and steel products, detection for a defect existing in a product according to an ultrasonic testing and determination on whether or not it is acceptable are carried out. The standard for determining whether or not any product is acceptable is specified by for example, the dimensions of a defect which should be detected. For example, according to API standard 5CT, which is one of Oil Country Tubular Goods (OCTG) related standards, it is stipulated that if any defect which surface is not open within the steel pipe or tube (which is not exposed on the inner and outer surfaces of the steel pipe or tube) is detected, the area of that defect shall not be 260 mm2 or more (API Specification 5CT/ISO 11960). The area of the defect is an important factor for guaranteeing the quality of a product.
As a conventional method for calculating the area of any defect quantitatively by ultrasonic testing, there have been known (A) a method for calculating the area of the defect using the height of echo and (B) a method for calculating the area of the defect according to a moving distance in which defect echo appears when an ultrasonic probe is moved, as described in Non-Patent Literature 1 (“Ultrasonic Flaw Detection Test III” 2001, compiled by the Japanese Society for Non-Destructive Inspection, Jun. 11, 2001, pp. 57-58 and pp. 117-118).
Further, (C) a method for calculating the area of the defect using aperture synthetic processing has been proposed in Patent Literature 1 (Japanese Laid-Open Patent Publication No. 2005-31061). Hereinafter, these methods will be described in detail.
(A) Method for Calculating the Area of a Defect Using the Height of Echo
When the size of a defect is smaller than the effective width of ultrasonic beam, the area of the defect can be calculated using a relationship that the area of the defect and the height of echo are proportional to each other.
An echo height PR from a circular flat defect existing at a point by a distance (distance on the central axis of a circular transducer) x1 from the circular transducer which constitutes the ultrasonic probe is expressed in the following equation (1).
                    ⁢          [              Equation        ⁢                                  ⁢        1            ]                                                                    ⁢                                    P              R                        ≈                                          P                O                            ·                                                π                  ⁢                                                                          ⁢                                      D                    2                                                                    4                  ⁢                  λ                  ⁢                                                                          ⁢                                      x                    1                                                              ·                                                π                  ⁢                                                                          ⁢                                      d                    2                                                                    4                  ⁢                  λ                  ⁢                                                                          ⁢                                      x                    1                                                                                                            (          1          )                    
where, in the above equation (1), P0 means incident sound pressure of ultrasonic wave, λ means the wavelength of ultrasonic wave, D means the diameter of a transducer and d means the diameter of a defect.
It is evident from the above equation (1) that the echo height PR of the defect is proportional to πd2/4 which is the area of the defect.
On the other hand, if a test object has a sufficiently wide plane as its bottom surface, the echo height P∞ of the bottom surface is expressed in the following equation (2) when a distance from the transducer to the bottom surface is x2.
                                              ⁢                  [                      Equation            ⁢                                                  ⁢            2                    ]                                                                                              ⁢                              P            ∞                    ≈                                    P              O                        ·                                          π                ⁢                                                                  ⁢                                  D                  2                                                            8                ⁢                λ                ⁢                                                                  ⁢                                  x                  2                                                                                        (        2        )            
The area of the defect can be estimated by obtaining the ratio of the echo height P∞ of the bottom surface and the echo height PR of the defect according to the above equations (1) and (2) and measuring the distances x1, x2.
However, the above equation (1) is established when the surface of a defect is parallel to the surface of the transducer of the ultrasonic probe. In other words, the calculation method is based on an assumption that the surface of the defect is parallel to the surface of the transducer of the ultrasonic probe and that a maximum echo from the defect is received by the ultrasonic probe. Therefore, if the surface of the defect is tilted with respect to the surface of the transducer, the echo reflected by the defect is hard to receive by the ultrasonic probe thereby reducing its calculation accuracy, which is a problem. Further, the calculation method cannot be employed if the size of the defect is larger than the effective width of ultrasonic beam. Thus, if the test object is a rolled material such as a steel pipe or tube and a steel sheet, it is necessary to use an ultrasonic probe having a large transducer for a planar defect extending in the rolling direction, which is not realistic.
(B) Method for Calculating the Area of a Defect According to a Moving Distance in which Defect Echo Appears by Moving the Ultrasonic Probe
In a case where the size of a defect is larger than the effective width of ultrasonic beam, there have been known a method for measuring a range in which the defect echo drops from a maximum echo height to a predetermined level by moving the ultrasonic probe or measuring a range in which the echo height appears over a predetermined height regardless of the maximum echo height, as an indicative length of the defect. According to this method, the length of the defect can be measured with relatively high precision by selecting an ultrasonic probe having smaller transducers than the length of the defect which is a measuring object. As for the planar defect extending in the rolling direction of a rolled material such as a steel pipe or tube and a steel sheet and the like, defect dimensions (defect length) in the rolling direction can be measured with relatively high precision according to the method.
However, it is difficult to satisfy the prerequisite of the method that the size of the defect is larger than the effective width of ultrasonic beam since the dimension of the defect (defect width) in a direction perpendicular to the rolling direction is smaller than that in the rolling direction. The reason is that if the dimension of the transducer is reduced, ultrasonic beam is expanded and if the dimension of the transducer is increased, oscillated ultrasonic beam itself is expanded.
FIG. 1 shows an example of a result of measuring the echo height by moving ultrasonic probes whose the width of transducers are 3.5 mm and 0.7 mm, respectively, with respect to a defect of width 1 mm in its defect width direction at a position apart by 10 mm from the defect (with the ultrasonic probe installed on the surface of a test object in which the defect exists at a position of 10 mm in depth, moving in the defect width direction). As shown in FIG. 1, in a case where any ultrasonic probe is used, the distribution of the echo height in the width direction exhibits a smooth shape originating from a fact that the effective width of the ultrasonic beam is large. If the range in which the echo height drops by 6 dB from the maximum echo height is assumed as a defect width, the defect width to be measured by each ultrasonic probe is 6.3 mm and 2.8 mm, which is larger than an actual defect width (1 mm).
Therefore, In the above method, even if the length of any planar defect extending in the rolling direction, having a small width in a direction perpendicular to the rolling direction of a rolled material such as a steel pipe or tube and a steel sheet can be measured with relatively high precision, the defect width is measured to be larger than its actual width. That is, the above method calculates the area of the defect to be excessive. As a result, any product which is not actually defective is determined to be defective thereby possibly reducing the yield.
(C) Method for Calculating the Area of a Defect by Aperture Synthetic Processing.
On the other hand, Patent Literature 1 has disclosed a method in which three-dimensional imaging data of the interior of a test object is generated based on data collected by executing ultrasonic flaw detection using a group of transducers arranged in a matrix state and then this three-dimensional imaging data is processed to automatically calculate the area of the defect. More specifically, when the area of the defect is automatically calculated from the three-dimensional imaging data, the three-dimensional imaging data is seen through in each axial direction of orthogonal coordinates to project data having a maximum value to a plane. Then, by counting the number of meshes having a higher value than a predetermined threshold on the projection plane, the area of the defect is calculated. This method enables the defect to be displayed at high resolution by employing the aperture synthetic technique when any three-dimensional imaging data is generated. However, there is a problem in calculation efficiency and calculation accuracy when this method is applied to the planar defect or the like of the rolled material. Hereinafter, this method will be described in detail.
It has been known that the resolution of the aperture synthetic-processing image obtained by the aperture synthetic processing depends on an arrangement pitch of the transducer and the size of the aperture. The size of the aperture is similar to the entire dimension of the group of the transducers which receive an echo at the time of the aperture synthetic processing (entire dimension of the group of the transducers arranged in a direction in which the aperture synthetic processing is carried out). Then, it has been known that the smaller the arrangement pitch of the transducers and the larger the size of the aperture (entire dimension of the group of the transducers), the higher the resolution becomes.
Therefore, it can be expected that the dimension of the defect is measured in the aforementioned direction with high precision by using the group of the transducers, the group being configured by arranging a number of the transducers each having a minute dimension in a direction in which the dimension of the defect is required to be measured with high precision. However, the number of the transducers which can be disposed is limited from the perspective of equipment cost, because an electronic circuit relating to exchange and processing of signals is connected to each of these transducers and such an equipment prevalent currently contains about 256 transducers.
As described above, to calculate the area of a defect in a rolled material, it is necessary to measure dimensions of the defect in a direction orthogonal to the rolling direction with high precision because the defect is long in the rolling direction while it is short in the direction orthogonal to the rolling direction, in order to enhance the accuracy of calculation on the area of the defect. When a group of transducers in which the transducers having a minute dimension are arranged densely in a matrix state is used, the resolution in the rolling direction is intensified more than required, while the resolution in the direction orthogonal to the rolling direction is dropped because the size of the aperture is decreased and further, a range which can be measured all at once becomes narrow. For example, to obtain a resolution of about 0.3 mm, at least the arrangement pitch of the transducers needs to be about 0.6 mm, because the arrangement pitch of the transducers needs to be twice or more the resolution in the aperture synthetic processing. When the arrangement pitch of the transducers is 0.6 mm, the size of the aperture is about 0.6×16=9.6 mm in the case of the group of the transducers in which 16×16 (=256) transducers are arranged in a matrix state. Further, the resolution at a point of a predetermined depth of a test object just below the center of the group of the transducers is assumed to be λ/(2 sin θ) when the aperture angle is 2θ and the wavelength of ultrasonic wave is λ. For example, a steel material having a sound velocity of 5960 m/s is employed as a test object and a group of the transducers is placed on the surface of the test object. If the ultrasonic testing frequency is set to 5 MHz, the resolution at a depth of 10 mm from the surface of steel material is about 1.4 mm, indicating that the resolution is dropped. Additionally, a range in which the measurements in the direction orthogonal to the rolling direction can be done all at once is reduced to about 9.6 mm which is similar to the size of an aperture. Therefore, it is difficult to say that the calculation efficiency and calculation accuracy have a balance with each other.